1. Field of the Invention
The present invention relates to a semiconductor laser device, and particularly to a semiconductor laser device having a window structure formed therein.
2. Description of the Related Art
In a semiconductor laser device, a current density at its edge facets increases under high power oscillation, so that an optical density increases and breakage and the like occurs at the edge facets, thereby making it difficult to obtain high reliability. Thus, there have been made many proposals concerning a structure, so-called a window structure, for providing current non-injection regions in the vicinities of the edge facets of a resonator.
In the gazette of Japanese Unexamined Patent Publication No. 2000-31596 is, for example, a window structure fabricated in such a manner that a cladding layer disclosed is etched to the vicinity of a quantum-well active layer and a re-grown cladding layer added with dopant is formed, whereby the dopant is diffused to the quantum-well layer and is subjected to mixed crystallization. However, the window structure described above has a problem such that variations of thermal diffusion in a regrowth process make the depth of diffusion and the mixed crystallization uneven, deteriorating reproducibility of the window structure. Thus, in high power oscillation, it has been difficult to obtain a highly reliable semiconductor laser at high yields. Furthermore, in the constitution described above, three crystal growth processes and two dry etching processes must be performed in order to fabricate an element. Thus, there is concern for complication in actual manufacture thereof and for increase in costs.
Meanwhile, in Japanese Unexamined Patent Publication No. 11 (1999)-298086, a semiconductor laser device is proposed, which is capable of increasing a bandgap and of reducing light absorption at end facets of the device. Such an increase in the bandgap is realized by lattice relaxation in the vicinity of an active layer, the lattice relaxation being attributable to adoption of a semiconductor layer constitution having strain compensation incorporated in the periphery of the active layer. With the above semiconductor layer constitution, element separation by cleavage causes lattice relaxation at an edge of light emission and increases the bandgap, whereby a window portion is naturally formed. However, in the foregoing semiconductor laser device, since current injection regions extending to end facets of the device are formed, there is a problem that a complete window structure cannot be formed.
An object of the present invention, in consideration for the above circumstances, is to provide a semiconductor laser device provided with a precise window structure which can be manufactured in a simple process and at high yields.
A semiconductor laser device of the present invention comprises a GaAs substrate, a cladding layer of a first conductivity type formed on the GaAs substrate, a first optical waveguide layer formed on the cladding layer of the first conductivity type, a first GaAs1xe2x88x92y2Py2 barrier layer formed on the first optical waveguide layer, an Inx3Ga1xe2x88x92x3As1xe2x88x92y3Py3 quantum-well active layer formed on the first GaAs1xe2x88x92y2Py2 barrier layer, a second GaAs1xe2x88x92y2Py2 barrier layer formed on the Inx3Ga1xe2x88x92x3As1xe2x88x92y3Py3 quantum-well active layer, a second optical waveguide layer formed on the second GaAs1xe2x88x92y2Py2 barrier layer, a cladding layer of a second conductivity type formed on the second optical waveguide layer, a GaAs contact layer formed on the cladding layer of the second conductivity type and an electrode formed on the GaAs contact layer. Each of the cladding layers of the first and second conductivity types has a composition which lattice-matches with the GaAs substrate. Each of the first and second optical waveguide layers has an InGaAsP series composition which lattice-matches with the GaAs substrate. Each of the first and second barrier layers is a layer which has a tensile strain relative to the GaAs substrate and has a layer thickness of 10 to 30 nm. Each barrier layer has a composition which satisfies a condition that the product of a strain amount of the tensile strain and the layer thickness of the barrier layer is in the range of 5 to 20% nm. The Inx3Ga1xe2x88x92x3As1xe2x88x92y3Py3 quantum-well active layer is a layer having a layer thickness in the range of 6 to 10 nm and has a composition having a compressive strain of 1.0% or more with respective to the GaAs substrate. The absolute value of the sum of the product of the strain amount and layer thickness of the first barrier layer and the product of the strain amount and layer thickness of the second barrier layer is greater than the absolute value of the product of the strain amount and layer thickness of the quantum-well active layer. The semiconductor laser device has a current non-injection region formed in a portion including at least one of the resonator edge facets.
The current non-injection region is preferably formed in a region where a distance extending from the edge facet toward the inner portion of the resonator is in the range of 5 xcexcm to 50 xcexcm.
A current non-injection region may also be formed by forming the GaAs contact layer in a place on the cladding layer of the second conductivity type other than the current non-injection region.
Besides the above, a current non-injection region may also be formed by providing an insulating layer or the like in the vicinity of the end facet between the active layer and the contact layer.
Moreover, in the foregoing semiconductor laser device, a current confinement layer of a first conductivity type may be provided between the second optical waveguide layer and the cladding layer of the second conductivity type.
Here, the first conductivity type and the second conductivity type are opposite to each other. For example, when the first conductivity type is a p-type, the second conductivity type is an n-type.
Compared with a conventional semiconductor laser with a 1.0 xcexcm band, which includes aluminum (Al) in an active layer, the semiconductor laser device of the present invention has high reliability regarding durability because the active layer is constituted by a composition including no aluminum (Al). Moreover, the semiconductor laser device of the present invention is capable of increasing a bandgap by lattice relaxation in the vicinity of the active layer, which is attributable to the GaAsP tensile-strain barrier layers provided therein, and is capable of reducing light absorption at the light emission edge facet of the device. Furthermore, with respect to the active layer having the compressive strain approximate to a critical film thickness during crystal growth, tensile strains are added to the first and second barrier layers to compensate for a part of the strain of the active layer. Thus, a high-quality active layer can be formed. In addition, surface diffusion of indium (In) caused during the growth can be suppressed by the existence of the GaAsP layer, whereby a high-quality crystal can be obtained. Moreover, the GaAsP tensile-strain barrier layers increase the height of walls between the active layer and the barrier layers, whereby leakage of electrons and electron holes from the active layer to the optical waveguide layers can be reduced. Owing to the effects as described above, driving currents can be reduced, and generation of heat at the edge facets of the device can be reduced. Furthermore, since the current non-injection regions are provided in the vicinities of the resonator edge facets, currents are not injected into the vicinities of the resonator edge facets, whereby current densities at the edge facets can be reduced and generation of heat at the edge facets can be further reduced. Accordingly, breakage of the edge facets and the like due to increase in an optical density can be suppressed. Therefore, highly reliable beams can be obtained from a low-power operation through a higher-power operation.
If the current non-injection region is constructed by forming the GaAs contact layer in a place on the cladding layer of the second conductivity type other than the current non-injection region, the current non-injection region can be constructed very easily and surely.